The present invention relates to a MOS technology circuit of substantially constant transconductance and, in particular, to a transconductance circuit having at least one transconductance subcircuit, which subcircuit is connected between two supply terminals and includes a MOS transistor. Transconductance circuits of this kind, which are often called voltage/current converters, are widely used in analog integrated circuits and particularly in integrating circuits to produce, for example, filters, oscillators and delay circuits.
Transconductance circuits of this kind may comprise active circuits and polysilicon or diffused resistors R and their transconductance Gm is a function of the ratio 1/R. However, the value of the resistance R varies with temperature, which makes the value of the transconductance unstable. What is more, the value of the resistance depends on the manufacturing process. The tolerance on the value of the resistance is of the order of plus/minus 15 to 20% and this is reflected in the transconductance.
Transconductance circuits produced by bipolar or MOS techniques have a transconductance Gm that is proportional to I/VT or I/2Vgt respectively, where I is the output current from the transconductance circuit and VT is the threshold voltage and Vgt the gate overdrive voltage of a MOS transistor. The transconductance Gm varies, in particular, as the current and the latter is not constant and depends on the one hand on temperature and on the other on the manufacturing process.
However, particularly in transconductance integrating circuits, efforts are made to keep the transconductance Gm substantially constant because it is on the value of the transconductance that the time constant of the circuit depends. Such integrating circuits do in fact comprise at least one transconductance circuit of transconductance Gm, and at least one integrating capacitor of capacitance C connected to the output of the transconductance circuit, and their time constant T is proportional to the ratio C/Gm. It is important for the time constant T to be as constant as possible in a large number of applications. Efforts are also made to enable the time constant to be accurately known and thus to be as insensitive as possible to the process by which the integrating circuit is manufactured.
To make the time constant substantially constant, it is necessary to subject it to feedback control. The value of the time constant is measured and, if it is different than a desired value, it is corrected. The feedback control circuit requires a reference clock signal, counters, a phase detection circuit or a phase-lock loop circuit to make the measurement and a network of resistors and capacitors to make the correction. This feedback control circuit causes a by no means negligible increase in the cost of the integrating circuit, in its energy consumption and in its size.
The present invention aims precisely to produce, in a simple way, a MOS technology transconductance circuit whose transconductance is substantially constant.
In a circuit of this type, the transconductance is seen to depend on, among other things, the mobility xcexc of the majority carriers (electrons or holes depending on the type of MOS transistor) in the channel of the MOS transistor and this value varies greatly with temperature. The idea that is followed to make the transconductance substantially constant is to compensate for the thermal variations in the mobility I of the majority carriers.
To achieve this, the present invention proposes a transconductance circuit having at least one transconductance subcircuit, which subcircuit is connected between two supply terminals and includes at least one MOS transistor. The circuit comprises means for biasing the MOS transistor in the subcircuit with a biasing current whose variation as a function of temperature substantially compensates for that of the mobility of the majority carriers in the channel of the MOS transistor in the subcircuit, in such a way as to make the transconductance of the circuit substantially independent of temperature.
The biasing means may comprise a current mirror connected to the MOS transistor in the subcircuit, this current mirror cooperating with a tuning circuit that is connected in turn to a reference-voltage generator, the tuning circuit comprising a MOS tuning transistor through which the biasing current that the current mirror duplicates flows, and the gate overdrive voltage of the MOS tuning transistor having a gradient with temperature that is substantially equal and opposite to that of the majority carriers in the channel of the MOS transistor in the subcircuit, said gate overdrive voltage being obtained from the reference-voltage generator.
The tuning circuit may also comprise a bipolar transistor whose emitter is connected to one of the supply terminals via a resistor, whose base is connected to the reference-voltage generator and whose collector is connected on the one hand to the other supply terminal via a series circuit having a diode and a resistor and on the other hand to the gate of the MOS tuning transistor that is connected between the other supply terminal and the current mirror.
The reference-voltage generator is intended to supply the tuning circuit with a reference voltage that enables a gradient with temperature to be obtained such that the gradient with temperature of the gate overdrive voltage of the MOS tuning transistor substantially compensates for that of the mobility of the majority carriers in the MOS transistor in the subcircuit.
Any conventional reference-voltage generator, such as, for example, a conventional generator of a reference voltage based on the forbidden energy band of a semiconductor material, may be used to produce a reference-voltage generator having the above characteristics. The voltage produced by a conventional generator of this kind has a given temperature dependence, generally of between 0 and 1. However, the temperature dependence of the gate overdrive voltage of the MOS tuning transistor may be altered in accordance with the invention to substantially compensate for that of the mobility of the majority carriers in the MOS transistor in the subcircuit.
For this purpose the conventional generator is connected to a divider bridge that includes, for example, two resistors, one of the two being connected to the output of the conventional generator and the other to ground, the center point between these two resistors being connected to the input of the tuning circuit, i.e. to the base of the tuning transistor. By altering the relative values of the resistors and hence the value of the voltage at the center point, the gradient with temperature at the emitter of the tuning transistor may be altered. Thus the combination of a conventional reference-voltage generator and a voltage divider bridge enables the gradient with temperature of the gate overdrive voltage of the MOS tuning transistor to be caused to substantially compensate for that of the mobility of the majority carriers in the MOS transistor in the subcircuit.
It is also possible for direct use to be made of a reference-voltage generator that gives a voltage enabling a desired, controlled temperature dependence to be obtained in the tuning transistor. An example of such a generator will be described.
The transconductance subcircuit may comprise a differential pair of MOS transistors whose gates form the inputs of the transconductance circuit and whose drains form its outputs.
For the purposes of linearization, the differential pair of MOS transistors may cooperate with a degeneracy resistor that is connected between the sources of the MOS transistors making up the pair.
The degeneracy resistor may be formed by a pair of MOS transistors that each have their gates connected to the gates of respective ones of the MOS transistors making up the differential pair.
The transconductance subcircuit may be connected between the two supply terminals via the biasing means on one side and a load circuit on the other.
The load circuit may be passive.
In another embodiment, the load circuit may be formed on the basis of a current source that cooperates with a system for the common mode feedback control of the outputs of the transconductance circuit.
Another object of the invention is to produce an integrating circuit from the foregoing circuit and to makes its time constant substantially independent of temperature and the manufacturing process. An integrating circuit of this kind does not need a circuit for feedback control of the time constant. An integrating circuit of this kind comprises at least one transconductance circuit as defined above, whose output is connected to an integrating capacitor produced on a MOS transistor basis.
The present invention also relates to a filter that comprises at least one such integrating circuit.
The present invention also relates to a delay circuit or an oscillator that comprises at least one such integrating circuit. The invention may thus be applied in an apparatus intended for the reception and transmission of radio telecommunications signals that includes a transconductance circuit according to the invention. An apparatus of this kind may be a telephone for example.